In the graph we see that when we increase the mass of baking soda, the volume of CO2 formed increases. Since there will be more reagent to react.
But there comes a mass when the slope flattens out and presents a similar volume even if the mass of baking soda is increased.
This may be because the other reactant becomes a limiting reactant, so the gas production will no longer depend on the amount of mass of baking soda but on the other reactant. That is why you see two slopes with different behaviors.